1. Field of the Invention
The present invention relates to a rear bicycle derailleur to be used as a rear exterior chain shifting device (derailleur) of a bicycle.
2. Description of the Related Art
As the exterior chain shifting device (derailleur) for a bicycle, a rear bicycle derailleur is used. The rear derailleur is a device for speed-changing the bicycle by shifting a chain which is looped over one of a plurality of sprockets or chain wheels arranged concentrically adjacent another sprocket or chain wheel. An example of the derailleur which has been mainly used for the bicycle currently has been disclosed in a number of patent literatures such as WO96/24787 and Japanese Patent Laid-Open No. 2000-247284.
Such a derailleur, particularly the rear derailleur is provided with a tension arm, a freely-pivotable base portion biased in one direction of rotation, and which is biased in the opposite direction by a parallel rocking member or movable member through a pantograph mechanism (i.e., a parallel link mechanism). Two sprocket wheels or pulleys are rotatably supported by this tension arm. A chain is looped over these two sprocket wheels (guide sprocket wheel and tension sprocket wheel) and the chain wheel. A guide sprocket wheel side closer to the above-described chain wheel is moved toward the front of any one of the above-described plurality of chain wheels or sprockets (at this time, the tension sprocket also moves together with guide sprocket) by the above-described pantograph mechanism (i.e., parallel link mechanism), thereby shifting the chain.
Sag or slack of the chain caused by the replacement (shift) is absorbed by combination of two pivot movement in which the pantograph mechanism and the parallel rocking member (movable member) simultaneously pivot and the tension arm provided at a tip end of the movable member also pivots. For this reason, the operation is complicated and this traditional design has suffered from some problems in terms of balance, durability and the like.
Also, parts of bicycles have been standardized. A user typically selects any appropriate wheels from among many sizes in accordance with rider size or a site of use for use in the same manner as other parts. At this time, since some rear derailleurs cannot cope with small wheels, wheels of a desired size could not be selected sometimes.